A Study of the Conversion Kinetics of High-Viscosity Oil Components During Ultrasonic Treatment in the Presence of Zeolite

Abstract

In this work, the kinetics of the redistribution of oils, resins, and asphaltenes in high-viscosity oil from the Karazhanbas field (Republic of Kazakhstan) were investigated. This was achieved with an ultrasonic treatment (22 kHz, 50 W) in the presence of a zeolite catalyst (1.0 wt%). The parameters of the technological process were established as a temperature range from 30 to 70 °C and an exposure time of 3 to 11 min. This allowed us to increase the oil content by 14.8% and decrease the concentration of resins by 12.2% and asphaltenes by 2.6%. Conversion schemes (“oils ↔ resins” and “resins ↔ asphaltenes”) were developed, which made it possible to determine the main direction of the reaction processes. The most rapid process is the conversion of resins to oils (k₂ = 0.1148–0.1860 min⁻¹). The process of the cracking of asphaltenes with the formation of resins (k₄ = 0.1023–0.1413 min⁻¹) ranks second in rates. Condensation reactions, including the transition of oils to resins (k₁ = 0.0175–0.0252 min⁻¹) and resins to asphaltenes (k₃ = 0.0139–0.0194 min⁻¹), occur significantly more slowly. The calculated activation energies (7.0–10.4 kJ/mol) show that the cavitation treatment of high-viscosity oil in the presence of a catalyst effectuates the processing of heavy oil with minimal energy consumption. A group composition analysis of the light and middle oil fractions demonstrated an increase in paraffinic, naphthenic, benzenic, and olefinic hydrocarbons, with a simultaneous decrease in naphthalenes and heteroatomic compounds. The results obtained confirm the effectiveness of ultrasonic–catalytic treatment for the structural cracking of high-viscosity oil and the formation of lighter hydrocarbon fractions.

1. Introduction

The issues of the preliminary processing of heavy oil and unconventional oil types are particularly relevant due to a growing trend of declining light oil production, as well as the growth of the global energy demand. In this regard, ultrasonic processing methods have become one of the most important areas of research [1,2,3,4]. Over the past decades, ultrasonic technology has moved from basic applications in the separation of oil and water emulsions to advanced methods of viscosity reduction and the chemical transformation of heavy oil components, including desulfurization and dementalization [5,6,7,8,9]. This area of research is of practical importance for improving oil recovery, optimizing transportation and improving refining processes. The economic and environmental benefits of ultrasound treatment highlight its growing importance in petroleum engineering [10,11].

Ultrasound induces cavitation bubbles, the collapse of which generates localized energy that is sufficient to break carbon–carbon and carbon–heteroatom bonds in asphaltenes and resins. This process leads to molecular fragmentation, alterations in the group composition, and a reduction in the size of asphaltene aggregates [12,13,14]. These chemical changes correlate with reduced viscosity and improved fluidity, establishing a mechanistic relationship between ultrasonic parameters, chemical transformations, and the macroscopic properties of oil [15].

A number of authors note the predominantly destructive effect of ultrasound. Thus, Razavifar et al. [16] demonstrated a decrease in the average molecular weight of heavy oil by ~7% and a decrease in the size of asphaltene aggregates, which was accompanied by a decrease in the viscosity of the system. According to Shi et al. [17], this viscosity reduction amounts to up to 60%. However, the presence of paraffin complicates this process. The ultrasonic treatment of the oil promotes paraffin crystallization, leading to an increase in the viscosity, pour point, and the amount of deposited paraffin [18]. In addition, the viscosity may initially decrease but tends to increase again over time as the oil relaxes, as observed in [5]. Studies [6] report a contradictory effect on the content and structure of asphaltenes, which indicates an incomplete understanding of the effect of ultrasound on molecular associations. In addition, the influence of ultrasonic parameters, such as frequency, power and exposure time, on changes in the chemical composition are discussed in [19]. In the article, Dengaev et al [1] emphasize that, despite the widespread use of ultrasound in oil refining, the chemical transformation of hydrocarbons, especially in heavy oil, remains insufficiently researched.

Numerous studies have shown that ultrasonic cavitation contributes to the cracking of high-molecular-weight compounds, the disaggregation of asphaltenes and a decrease in the viscosity of heavy oils. However, a study [20] concluded that a high-frequency ultrasonic field treatment significantly alters the physical and chemical characteristics of high-paraffin crude oil, leading to an increase in viscosity and related mechanical properties. This is due to the fact that without stabilizing factors, radical processes under the influence of ultrasonic cavitation are accompanied by both cracking and secondary polycondensation. Radical processes can be stabilized by the addition of hydrogen, hydrogen donors, aromatic solvents, or catalysts [13]. The integration of ultrasound and catalysts has been shown to promote the splitting of complex molecules. This results in a reduction in tar and asphaltenes and an increase in hydrogen-to-carbon ratios, leading to a lower oil viscosity and an improvement in the overall oil quality [7,21]. The application of a Ni skeletal catalyst together with ultrasonic treatment enhanced the quality of crude oil from the Zhanazhol field, leading to a significant increase in gasoline and diesel yields and a 49% reduction in the sulfur content [22].

This data confirms the potential of the sonocatalytic approach for processing high-viscosity oils. In this regard, the aim of this work is to study the kinetics of the group composition redistribution (oil, resin, and asphaltenes) during the ultrasonic treatment of high-viscosity oil in the presence of acid zeolites.

2. Materials and Methods

Heavy oil from the Karazhanbas field (Kazakhstan) was subjected to ultrasonic treatment. The physical and chemical properties of the oil are shown in

Table 1. Physical and chemical characteristics of high-viscosity oil from the Karazhanbas field.

Indicators	Values
Density, g/cm3
20 °C	0.935
Dynamic viscosity at 30 °C, mPa·s
20 °C	1350
Content, wt. %
Oil	68.8
Resin	25.1
Asphaltenes	6.1
Solid paraffins with melting points higher than 42 °C	1.5
Elemental composition, %
C	82.6
H	11.8
N	0.6
S	2.4
O	2.6
Fraction yield, %
Up to 200 °C	3.6
200–300 °C	14.7
Metal content, g/t
Vanadium	215
Nickel	57

.

As the data presented indicates, the high content of resins and asphaltenes (31.2%) determines the colloidal stability of the system and forms the increased viscosity of the raw material. At the same time, the elemental composition, characterized by increased concentrations of oxygen, sulfur, nitrogen, as well as metals (V and Ni), significantly complicates catalytic processing [23,24]. In this regard, in modern practice, special attention is paid to the development and application of methods for the preliminary activation of high-viscosity oils with the aim of increasing the yield of light distillate fractions during subsequent processing stages [25,26].

Synthetic acid Y-zeolite (supplied by Snabtekhmet (Almaty, Republic of Kazakhstan)) was used as the catalyst for the ultrasonic treatment of high-viscosity oil. The zeolite (20 g) was activated with a 20% hydrochloric acid (HCl) solution (300 mL) at 60–70 °C. After acid treatment, the zeolite was washed with distilled water until a neutral value was reached, then separated by filtration, and dried at 105 °C for 2 h. The specific surface area and pore volume of the sample were determined by BET and STSA methods using nitrogen as an adsorbate gas on a SORBOMETER-M analyzer (Katakon, Russia) (

Table 2. Textural characteristics of zeolite.

Sample	SBET, (m2/g)	Smesoporous, (m2/g)	SMICRO, (m2/g)	VTOTAL, (cm3/g)	Vmesoporous, (cm3/g)	VMICRO, (cm3/g)	Pore Size, nm
Y-zeolite	630.7	145.1	485.6	0.310	0.065	0.245	2.005

).

The high catalytic efficiency of zeolites is attributed to the key factors for catalytic and adsorption processes, such as well-developed specific surface area (631 m²/g), pronounced acidity, and high thermal stability.

In addition to their porous structure, the acid sites provide an additional source of active hydrogen, which enhances the cracking of high-molecular-weight components while suppressing undesired polycondensation side reactions [27]. The acid properties of the catalysts were investigated using the temperature-programmed desorption of ammonia (NH₃-TPD) method: under standardized adsorption conditions, the catalyst surface was pre-saturated with ammonia molecules; subsequently, linear heating was carried out in an inert gas stream.

For zeolite-containing catalysts, the amount of ammonia desorbed at elevated temperatures is commonly used as an indicator of the concentration and strength of acid sites. For the studied catalyst, ammonia desorption amounted to 523.4 μmol/g at 175 °C and 1137.4 μmol/g at 245 °C. Zeolite has two types of acid sites, as evidenced by the presence of two forms of ammonia desorption on the thermodesorption spectrum: weakly acidic with a peak maximum temperature of T_max = 175 °C and strongly acidic with T_max = 245 °C. Weak acid sites are most often referred to as Lewis acid sites, which are incompletely coordinated aluminum atoms in the zeolite crystal lattice. Zeolite acid sites with high-temperature ammonia desorption temperatures are Brønsted acid sites, which are proton-donating OH groups associated with lattice Al [28,29].

For experimental studies, an ultrasonic device (model TEFIC-1000D, Xi’an, China) operating at a frequency of 22 kHz and an output power of 50 W was used. The duration of ultrasonic exposure varied between 3 and 11 min. The cavitation process was carried out in a 0.05 L glass reactor equipped with a thermostat maintaining temperature conditions of 30, 50, and 70 °C. The weight of the high-viscosity oil sample was 30.0 g, and the catalyst content was 1.0 wt% relative to the initial mass of oil.

The content of oils, resins, and asphaltenes in oil was determined by the standard method [30]. To isolate asphaltenes, the sample was diluted with 40 times the volume of hexane and kept for 24 h; the resulting precipitate was filtered off. The precipitate was placed in a paper cartridge and washed in a Soxhlet apparatus with hexane to remove oils and resins; the asphaltenes were washed out of the cartridge with chloroform. The diasphaltenized samples were applied to silica gel then were sequentially extracted in a Soxhlet apparatus into hydrocarbon components (oils), with n-hexane and resins isolated with a mixture of benzene and ethanol (1:1). Mass fractions were expressed in unit fractions of the initial oil sample.

To determine the group composition of oil fractions boiling up to 200 °C and 200–300 °C before and after ultrasonic treatment, chromatographic mass spectrometric analysis was performed on an Agilent Technologies 7890A (Santa Clara, CA, USA, MSD ChemStation E.2.00.493) gas chromatograph with a 5975C mass spectrometric detector.

3. Results and Discussion

A review of literary sources has shown that the studies on the ultrasonic processing kinetics of heavy hydrocarbons are limited. According to the data available [31,32], cavitation processes predominantly lead to the cracking and redistribution of high-molecular asphaltene–resin structures towards less condensed compounds. At the same time, deep carbonization or gasification is practically not observed. In [33], an investigation of the kinetics of the ultrasonic cavitation of the middle fraction of a coal resin was carried out. In it, the individual group of polyaromatic hydrocarbons were shown as components of the kinetic scheme. In [32], changes in the concentration of SARA fractions with increasing ultrasonic treatment durations were demonstrated; however, in that work, kinetic conclusions were obtained using the thermogravimetry method, which makes it impossible to directly compare their results with our kinetic model. To improve the quality of oil resins, the samples were subjected to an ultrasonic treatment at a power of 800 W, a temperature of 70 °C and an exposure duration of 0 to 11 min. It was found that the optimal treatment time is 7 min, at which the viscosity of the oil decreases by 14.1%, and the content of the coke residue decreases by 7.4%. At the same time, an increase in the proportion of saturated hydrocarbons and a decrease in the content of aromatic compounds, resins and asphaltenes were observed.

The proposed model for the transformation of high-viscosity oil under ultrasonic cavitation in the presence of a catalyst is based on the interconversion of its main fractional components: oils, resins, and asphaltenes. Thus, the kinetic model of heavy oil transformations was built based on the assumption of the existence of reversible transformation pathways between the three main fractions: oils (C1), resins (C2), and asphaltenes (C3) (Figure 1).

Based on Figure 1, a system of ordinary differential equations of the first order describing the rate of change in concentrations over time was created.

$${\displaystyle \frac{d{C}_1}{dt}}=-k_1{C}_1+k_2{C}_2$$

(1)

$${\displaystyle \frac{d{C}_2}{dt}}=k_1{C}_1-k_2{C}_2-k_3{C}_2+k_4{C}_3$$

(2)

$${\displaystyle \frac{d{C}_3}{dt}}=k_3{C}_2-k_4{C}_3$$

(3)

where

• C₁ is the oil fraction;
• C₂ is the resin fraction;
• C₃ is the asphaltene fraction;
• k is the rate constant.

To determine the kinetic parameters, kinetic curves of the process of the ultrasonic treatment of high-viscosity oil were taken at 30, 50 and 70 °C in a time interval from 3 to 11 min (Figure 2).

A consistent increase in the proportion of oils (C1) was observed, accompanied by a decrease in the concentrations of resins (C2) and asphaltenes (C3). The initial proportions of these components in the dehydrated oil are presented in Table 1. During the ultrasonic treatment of high-viscosity oil for 11 min, it was discovered that an increase in the process temperature contributes to more pronounced structural changes. Thus, at 30 °C the content of the oil fraction increases by 10.4%, at 50 °C by 14.3%, and at 70 °C it reaches 14.8%. Simultaneously, a decrease in the concentration of resinous substances was observed: by 8.4%, 12.1% and 12.2%, respectively. The proportion of asphaltenes decreases less significantly compared to resins: by 2.0%, 2.2% and 2.6%, respectively. The experimental error in determining the yields of oils, resins, and asphaltenes from high-viscosity oil amounted to ±3.4%. The observed increase in the oil content indicates partial radical cracking of high-molecular-weight compounds, leading to the formation of lighter fractions. Under these conditions, resins primarily act as intermediates: their accumulation does not occur; rather, significant depletion is observed due to the subsequent rapid transformation into oils. The preferential degradation of resin molecules under the ultrasonic treatment was demonstrated in previous studies, highlighting the effectiveness of ultrasonic waves in altering the chemical structure of these compounds [1]. It is highly likely that the acid sites of the zeolite catalyst play a key role in proton activation and the subsequent breakdown of resin–asphaltene aggregates. In addition, cavitation effects induced by ultrasonic exposure, including microjets, localized shear, and transient heating, contribute to the rupture of weak intermolecular bonds and the disaggregation of supramolecular structures. The average discrepancies between the experimentally measured concentrations of oil components and the values predicted by the model amounted to 0.78% at 30 °C, 1.30% at 50 °C, and 1.37% at 70 °C.

Table 3. Rate and energy constants for activation of transformations between high-viscosity oil components.

	Rate Constants
T, °C	k1, min−1	k2, min−1	k3, min−1	k4, min−1
30	0.0175	0.1148	0.0139	0.1023
50	0.0227	0.1691	0.0180	0.1213
70	0.0252	0.1860	0.0194	0.1413
	Activation energies, kJ/mol
	7.8	10.4	7.2	7.0
	Correlation coefficient, R2
	0.96	0.91	0.92	0.98

shows the kinetic parameters (rate and activation energy constants) for the conversion of components in high-viscosity oil.

The following parameters were used in the calculation of all rate constants: N_max = 100, step accuracy = 1.00 × 10⁻⁶, and criterion accuracy = 1.00 × 10⁻⁷. The average error at each point was 9.96 × 10⁻⁸ for the rate constants at 30 °C, 9.88 × 10⁻⁸ at 50 °C, and 9.94 × 10⁻⁸ at 70 °C. An analysis of the temperature dependences of the reaction rates shows an increase in the constants k₁–k₄ with an increase in temperature from 30 to 70 °C. The highest absolute values are characteristic of k₂ and k₄, which indicates the dominant role of the resin–oil (C2 → C1) and asphaltene–resin (C3 → C2) transitions in the overall transformation scheme.

The calculation of activation energies using the Arrhenius equation revealed values in the range of 7–10 kJ/mol. It should be emphasized that these values are significantly lower than typical activation energies for thermal processes of the cracking of resin–asphaltene structures [34]. The results obtained may indicate the combined catalytic and cavitation nature of the ongoing transformations. Local “hot spots” generated from cavitation, together with the acid sites of the zeolite catalyst, probably create conditions for more efficient reactions at relatively low energy barriers.

A comparison of activation energies shows that the highest energy barrier (≈10.4 kJ/mol) corresponds to the conversion of resins into oils (described by the rate constant k₂). This may represent the key limiting step in the degradation of resins to form oils. In contrast, lower activation energies for k₃ and k₄ (7.2 and 7.0 kJ/mol, respectively) indicate that transformations between resins and asphaltenes proceed relatively easily.

For a general qualitative analysis of the oil fraction composition, gas chromatography–mass spectrometry (GC–MS) was performed on the light and middle fractions of the high-viscosity oil before and after the ultrasonic treatment at 70 °C for 7 min in the presence of a zeolite catalyst.

Table 4. Group composition of light and middle fractions of high-viscosity oil.

Content	Before Treatment, wt%		After Treatment, wt%
	<200 °C	200–300	<200 °C	200–300 °C
Paraffins	13.42	26.03	22.48	63.44
Non-condensed naphthenes	31.82	4.85	37.17	5.06
Double ring condensed naphthenes	4.97	3.68	11.88	7.79
Benzene	6.09	3.78	7.11	4,36
Naphthalenes	8.96	14.87	1.32	5.43
Oxygen compounds	14.01	23.19	3.85	4.85
Nitrogenous compounds	2.91	3.74	-	-
Sulfur compounds	2.02	3.59	-	-
Alkynes	1.29	-	-	0.22
Olefins	3.12	1.97	7.81	6.21

presents the group composition of fractions boiling below 200 °C and in the 200–300 °C range.

A comparative analysis of the group composition of light (<200 °C) and middle (200–300 °C) fractions of high-viscosity oil before and after the ultrasonic treatment demonstrated significant changes in the distribution of hydrocarbon- and heteroatom-containing compounds. The most noticeable change is an increase in the proportion of paraffin hydrocarbons: from 13.42 to 22.48% in the light fraction and from 26.03 to 63.44% in the middle fraction. This indicates the cracking of heavier structures and the redistribution of products towards the paraffin phase.

The proportion of non-condensed naphthenes also increased (from 31.82 to 37.17% in the light fraction and from 4.85 to 5.06% in the middle fraction), which indicates a partial hydronaphthenization and stabilization of the products. At the same time, the concentration of condensed naphthenes increases even more significantly—from 4.97 to 11.88% and from 3.68 to 7.79% in the light and middle fraction, respectively—which can be associated with the restructuring of polycyclic structures.

Aromatic compounds exhibited multidirectional changes. The benzene content increased slightly (from 6.09 to 7.11% and from 3.78 to 4.36%), while naphthalenes decreased sharply (from 8.96 to 1.32% in the light fraction and from 14.87 to 5.43% in the middle fraction). This redistribution indicates the breakdown of more condensed aromatic structures and their partial conversion into less condensed or aliphatic products.

The content of oxygen-containing compounds decreased several times: from 14.01 to 3.85% and from 23.19 to 4.85%. Similarly, nitrogenous (2.91 and 3.74%) and sulfurous (2.02 and 3.59%) compounds completely disappeared, which indicates deep deasphaltenization and the cracking of resinous–asphaltene components.

Lastly, the proportion of olefins increased (from 3.12 to 7.81% and from 1.97 to 6.21%), which may result from partial dearomatization and the formation of unsaturated hydrocarbons during ultrasonic processing. At the same time, alkynes mostly disappear: their concentration in the light fraction decreases from 1.29% to an undetectable level. Only trace amounts of no more than 0.22% appear in the middle fraction.

Thus, the main trends include an increase in the proportion of paraffinic and olefinic compounds, a decrease in the content of heteroatom-containing components and a decrease in the concentration of polycyclic aromatic structures.

4. Conclusions

The results of this study confirmed the effectiveness of ultrasonic treatment combined with a zeolite catalyst for processing heavy (high-viscosity) oil to produce light fractions.

An analysis of the kinetics of the oil, resin, and asphaltene redistribution in high-viscosity Karazhanbas oil allowed us to establish the conditions under which the process proceeds with maximum efficiency. With an increase in temperature from 30 to 70 °C and a treatment duration of 3 to 11 min, a consistent increase in the oil content was accompanied by a simultaneous decrease in the concentrations of resins and asphaltenes. The highest rate of oil formation occurred within the first 7 min of the treatment.

The conversion schemes developed during this study (“oils ↔ resins” and “resins ↔ asphaltenes”) allowed for the identification of the dominant reaction pathways. The conversion of resins into oils occurred at the highest rate (k₂ = 0.1148–0.1860 min⁻¹). The cracking reaction of asphaltenes into resins (k₄ = 0.1023–0.1413 min⁻¹) is the second fastest conversion process. Condensation reactions of oils into resins (k₁ = 0.0175–0.0252 min⁻¹) and resins into asphaltenes (k₃ = 0.0139–0.0194 min⁻¹) proceed significantly more slowly. This confirms the impact of the ultrasonic exposure on the cracking of high-viscosity oil structures.

The activation energies calculated (7.0–10.4 kJ/mol) demonstrated that the cavitation treatment of high-viscosity oil in the presence of a catalyst allows the heavy oil to be processed at low energy costs. This indicates that the destructive transformations of the main components of high-viscosity oil occur primarily under the influence of the physicochemical effects of cavitation and not due to thermal decomposition at high temperatures.

A group analysis of light and middle oil fractions (after cavitation treatment at 70 °C for 7 min in the presence of 1.0% zeolite) revealed an increase in the content of paraffinic, naphthenic, benzene and olefinic hydrocarbons while reducing the proportion of naphthalene and heteroatomic compounds. The results obtained highlight new environmentally friendly opportunities for producing light fractions and confirm the effectiveness of ultrasonic treatment in the presence of a zeolite catalyst for processing high-viscosity oil. The kinetic parameters investigated in this study provide a basis for controlling the production process, thereby enhancing its economic efficiency.